Two types of handguns need no holster. I've been CC for 40 years, and I've found this to be true.
A slab-sided 1911 can be tucked in your pants over your right hip, cocked and locked. It will stay safely secure.
The same goes for 5-shot revolvers with rubber grips. The rubber prevents the gun from sliding too far down.
While carring this way, I've never experienced a mishap.
Even in the summer, if you can stand cold stainless steel against your skin, you can carry with no holster. Personally, I wear something very light over a tucked-in tee shirt.
Other guns must have a holster. A fat revolver or a fat auto, like a Sig, won't stay put in your pants without a holster.